I first replaced the two dol files, as suggested, no difference.
I compared the content of my SD card with the backup of the card that I have on my PC.
And I discovered that the USBLoader folder was missing in the Apps folder of my SD card.
So I copied the USBFolder to my SD card and the Wii is working again :)
Only difference from before is that the start up screen of the configurable USB Loader does not look the same as before. Never mind, it works again.
Strange however that a whole folder disappeared from the SD card. I am happy that it was an easy fix though, keeping all the game saves was important.
Thanks a great guide and for getting me back on the road!